1. Field of the Invention
The invention relates to a low thermal conductivity heat barrier coating, metal articles protected by the coating, and to a process for depositing the coating.
2. Summary of the Prior Art
For more than 30 years the makers of turbine engines for both land and aeronautical use have been tackling the problems of increasing the efficiency of such engines, reducing their specific fuel consumption, and reducing polluting emissions of the CO.sub.X, SO.sub.x and NO.sub.x type as well as unburnt constituents. One way of dealing with these demands is to get close to the combustion stoichiometry of the fuel and thus to increase the temperature of the gases leaving the combustion chamber and entering the first stages of the turbine.
It has therefore been necessary to adapt the turbine to this increase in combustion gas temperature. One solution adopted has been to improve the techniques for cooling the turbine blades, which has led to a considerable increase in the complexity and cost of the blades. Another solution has been to improve the refractory nature of the materials used in terms of peak working temperature, creep and fatigue life. This solution has been used since the development of the nickel and/or cobalt based superalloys, and underwent a considerable technical advance in the changeover from equiaxial superalloys to monocrystalline (single-crystal) superalloys, giving an 80 to 100.degree. C. improvement in creep resistance. Further developments along this route can only be made with substantial development costs, giving so-called third generation superalloys having an additional gain in creep resistance of about 20.degree. C. Beyond this a change in the family of materials is required.
An alternative to such a change is to deposit a thermally-insulating ceramic heat barrier coating on superalloy articles which are subjected to high temperatures. This ceramic coating makes it possible for a cooled article to have, during operation, a thermal gradient through the ceramic which is possibly in excess of 200.degree. C. The operating temperature of the metal below is reduced in proportion, with a considerable effect on the amount of cooling air required, the working life of the article, and the specific consumption of the engine.
The ceramic coating may be deposited on the article to be coated by various processes, most of which belong to two different categories, namely sprayed coatings and physically deposited vapour phase coatings. Other deposition processes of the chemical vapour phase kind assisted by plasma can also be used.
In the case of sprayed coatings a zirconia-based oxide is deposited by a technique related to plasma spraying. The coating consists of a stack of molten ceramic droplets which are then quenched, flattened and stacked to form an incompletely densified deposit of a thickness between 50 .mu.m and 1 mm. One of the characteristics of this kind of coating is intrinsically high roughness (the roughness Ra, being typically between 5 and 35 .mu.m). The degradation in service most often associated with this coating is the slow propagation of a crack in the ceramic parallel to the ceramic-to-metal interface.
The problem is quite different in the case of coatings deposited by physical vapour phase deposition. This deposition can be made by evaporation under electron bombardment. Its main characteristics is that the coating consists of an assembly of very thin columns (typically between 0.2 and 10 .mu.m) extending substantially perpendicularly to the surface to be coated. The thickness of such a coating can be between 20 and 600 .mu.m. An assembly of this kind has the useful property of reproducing without impairment the surface texture of the covered substrate. In particular, in the case of turbine blades a final roughness considerably less than 1 micrometer can be obtained, which is very advantageous for the aerodynamic properties of the blade. Another consequence of the columnar structure of physical vapour phase ceramic depositions is that the gap between the columns enables the coating to deal very effectively with compression stresses arising in operation due to differential expansion relative to the superalloy substrate. In this case long working lives can be achieved in respect of high-temperature heat fatigue, and the coating tends to rupture near the sublayer-ceramic interface.
Chemical vapour phase deposition techniques produce coatings whose morphology is columnar and substantially equivalent to that of physical vapour phase depositions. In both physical and chemical vapour phase depositions the oxide formation results from a molecular reaction between metal atoms or ions and oxygen.
Heat barrier coatings consist of a mixture of oxides, generally with a zirconia base. This oxide has a relatively low thermal conductivity and a relatively high coefficient of expansion close to that of the nickel and/or cobalt based alloys on which it is required to deposit the oxide. One of the most satisfactory ceramic compositions is zirconia totally or partially stabilized by an oxide such as, for example, yttrium oxide: ZrO.sub.2 +6 to 8 weight % of Y.sub.2 O.sub.3. The function of the yttrium oxide is to stabilize the cubic allotropic variety C and/or the non-transformable tetragonal variety t' of the zirconia and thus avoid martensitic phase transitions in swings between ambient temperature and the high operating temperature of the article.
The main function of a heat barrier coating is to slow down heat exchange between an external medium consisting of hot gases and the covered metal article, the latter usually being cooled by a forced flow of cold gases. Heat exchange between the ceramic coating and the metal below can be by conduction and, to some extent, by radiation. The thermal conductivity of the ceramic coating is the parameter which measures its effectiveness in slowing down heat conduction. Heat exchange by radiation is basically determined by the transparency or semi-transparency of the coating to the incident radiation, but the semi-transparency effect of the ceramic is of secondary importance in relation to conduction in the heat transfer process. Thermal conductivity is therefore the most important parameter which needs to be controlled to reduce heat transmission in the coatings.
There are a number of methods for reducing the thermal conductivity of the coating, and these are based on the fact that heat barrier coatings are porous ceramic layers and the thermal conductivity of the coating is that of a heterogeneous assembly of two heat-conducting media, namely the ceramic material itself with an intrinsic conductivity .lambda.intr, and the pores or microcracks of the coating whose conductivity is close to that of the air which fills them under operating conditions.
The effective conductivity .lambda.actual of the coating is between .lambda.intr and the conductivity of air .lambda.air. It can in fact be stated that .lambda.actual is a complex function of .lambda.intr, .lambda.air, and the morphology of the coating.
A first solution for obtaining a low thermal conductivity coating is to use a ceramic of a conventional ceramic composition, for example, zirconia partially stabilized by 6 to 8 weight % of yttrium oxide, and to modify the coating morphology--i.e., the proportion, distribution and orientation of the pores or microcracks of the coating, or the arrangement of the material in columnar or layer form so as to reduce .lambda.actual. This result can be achieved by modifying the coating deposition parameters.
A second solution is to modify the chemical composition of the coating in an attempt to reduce .lambda.intr directly without impairing the coating morphology, while retaining the other properties of the coating. This is the solution used in the present invention.